This invention relates generally to the treatment of the printed images from printing devices so as to improve their appearance. More particularly, it relates to the enhancement of color saturation, the protection of printed images from various types of damage and the extension of the lifetime of these images.
Digital printing involves the creation of graphical and textual material by means of depositing tiny spots of ink to a substrate. It differs from other forms of printing because the images are composed of well defined, uniformly placed spots of ink whose distribution and patterns are defined by the print engine rather than by photographic screens or monolithic type.
Ink jet printing is widely used for creating low cost, color printed output. Many aspects of the design, construction and operation of ink jet printers, and the chemical formulation of the inks used in these printers and also to the substrates upon which the printed image is created (e.g. coated papers, transparent materials for use in overhead projectors) have been taught in the prior art.
The quality of the output produced by ink Jet printers is limited by a number of factors related to the method used to produce it. There are a number of other printing technologies for producing color prints of higher perceived quality than ink jet, such as dye sublimation, thermal wax transfer and color xerography. Ink jet printed output is perceived to have reduced color intensity (a washed out appearance) because the ink must be carefully deposited so that adjacent pixels are distinct and separate. If not, then the inks will mix (xe2x80x9cbleedxe2x80x9d) and the resulting image will appear muddy and of low resolution.
FIG. 2 shows a series of the steps of an ink-jet printing process of the prior art. Element 202 is a drop of ink shown both in flight and upon contact with substratexe2x80x94204. Element 206 is this drop of ink xe2x80x9cimpingingxe2x80x9d onto the substrate. As depicted the droplet begins to lose its spherical shape as it encounters substrate 204. Element 208xe2x80x94is the same drop of ink, now being incorporated/absorbed into the substrate forming a wet drop of ink thus forming a spot. Element 210 depicts the spot somewhat later in time after the liquid vehicle of the ink has been completely absorbed into substrate 204. Finally, element 212 represents the spot after the spot has dried and the colorant immobilized into and/or atop substrate 204.
Jaeger et al.""s article entitled xe2x80x9cThe influence of Ink/Media interactions on Copy Quality in Ink-Jet Printingxe2x80x9d, published in Proceedings of the SID, Vol. 25/1, 1984, provides a comprehensive overview of the influence of ink and paper choices on the color quality of ink-jet prints. They conclude that the quality of these prints is dependent on the rapid absorption of ink into the substrate and the ability of the colorant to be retained on or near the surface of the substrate. They observe that photographic quality images require high-resolution prints (higher number dots per inch-dpi) and that the ultimate resolution of these prints is dependent on minimizing the lateral migration of the ink drop in the substrate. Thus their effort and focus is to prevent, minimize and limit the ultimate size of each dot applied to the printed image.
A great deal of effort has been directed toward improvements in reducing ink drop size by specially treated substrates that quickly absorb the wet ink spots, fast drying inks as well as combinations of inks that employ a variety of physical and chemical phenomena to reduce or eliminate ink mixing.
Palmer et al.""s article entitled xe2x80x9cink and Media Development for the HP Paintjet Printerxe2x80x9d, published in Hewlett-Packard Journal, August 1988, discusses the complex interactions between ink, print head design, properties of different substrates and the differing requirements of different forms of printing (text versus graphics). Their objective is to carefully control the resulting dot size to meet the requirements of each type of printing and each type of substrate (e.g. opaque versus transparent, coated versus non-coated). They clearly demonstrate that many factors must be considered (e.g. thermal tolerance of the ink, clogging of print heads, absorption of ambient moisture into coated papers thus making them tacky to the touch, etc.) when designing an Ink-jet printing system in which the dot size is carefully controlled to meet a variety of printing requirements.
Yoldas"" article entitled xe2x80x9cA dense transparent ink-jet receptive film that provides instantaneous print dryingxe2x80x9d, published in Journal of Material Research Vol. 14, No. 6, June 1999, focuses on the development of coatings for Ink-jet substrates that lead to a tighter containment of colorants resulting in higher edge acuity for deposited ink drops thus enabling high resolution images. Yoldas describes a family of Sol-Gel coatings that quickly absorb and immobilize the large volume of water used as the vehicle for colorant in ink-jet inks.
All of these teachings share the common objective of producing the smallest resulting ink dot on the printed substrate and the prevention of colorant migration and unintended mixing of different inks.
The undesired mixing of wet ink spots is generally called xe2x80x9cbleedingxe2x80x9d. One side effect of efforts to prevent bleeding is to deposit well-defined and separated ink spots. The substrate is not fully covered with ink and the underlying color of the substrate is seen. If that substrate is a white sheet of paper, then light scattered by the substrate itself increases the sense of the colors seeming to be xe2x80x9cwashed outxe2x80x9d.
A variety of methods have been suggested to overcome these limitations. These include making multiple printing passes over the substrate and depositing ink in patterns that overlap. Such methods require that the ink deposited on the first pass is sufficiently dry such that subsequent deposition of inks of other colors does not mix with the ink spots already deposited. Or it requires that specialized inks are used that are mutually immiscible so that colorants do not mix. These methods also require that more pixels are printed. Such methods increase the resolution of the image, but require a substantial increase (e.g. quadratic) in the number pixels to be printed. This requires the use of more ink, requires significantly longer printing times and because of very heavy ink loading, can cause the substrate material to wrinkle because of excess moisture.
U.S. Pat. No. 6,090,749 for Kowalski issued on Jul. 18, 2000 discloses a method for creating vivid and water-fast printed images on a specially designed multi-layer substrate. Kowalski requires the substrate be composed of two transparent layers: a hydrophilic, ink absorbing front layer and a hydrophobic backing layer. Kowalski also requires that the surface of the substrate is not covered or coated with any additional materials or layers. An ink jet printer is used to apply a highly specialized set of inks. These require use of xe2x80x9csublimable dye diffusion thermal transfer coloring agentxe2x80x9d. An ink jet printer is used to apply these inks onto an ink-absorbent hydrophilic front layer. The composite substrate is subsequently subjected to high temperature (and pressure) in order to vaporize the dye and to cause transference of colorant into a transparent hydrophobic backing layer. The transference of colorants into the interior of the backing layer is described as occurring by absorption, capillary action and alternatively termed diffused into the interior of the backing layer.
The objective of the Kowalski process is to produce a water-fast, vivid color image. These objectives impose a number of requirements and restrictions. For example, in order to assure that the resulting image is water-fast, all of the colorant initially deposited on the ink-absorbing layer must be transferred from the water absorbing front layer into the interior of the water resistant backing layer. Since all of the colorant must be removed from the first layer to the second layer, the transference of colorant cannot be color dependent.
The objective of creating a xe2x80x9cvividxe2x80x9d color image requires that lateral diffusion of colorant be restricted and ideally would not occur. Kowalski""s methods imposes similar restrictions on the initial application of inks as is necessary for simpler ink-Jet printing processes and then imposes additional requirements in the form of specially designed multi-layer substrates, specially designed inks and additional processes all of which are necessary to vaporize the colorants so as to transfer them into a transparent, water resistant backing layer.
There is a need, therefore, for methods for enhancing the perceived quality of the printed images and protecting the printed images from various types of damage.
Accordingly, an object of this invention is to provide methods for enhancing the perceived quality of ink jet printed output by a controlled and selective diffusion of the colorant in the deposited inks, and the encapsulation of the printed material so as to protect it from various types of damage (e.g., water spotting, bleaching of colorant).
It is a further object of the present invention to transform the well-defined spots into diffused, somewhat larger regions of color. Ideally, each pixel is transformed into a gaussian, graded distribution of colorant in which the center region of the deposited ink spot is of uniform color, but decreases in color intensity at the margins of the transformed pixel, which results in increased color intensity, reduced white light scattering off of the substrate and a more photographic, continuous tone appearance of the resulting printed output.
It is an additional object of the invention to provide encapsulation of the printed substrate with a protective coating that inhibits subsequent water spotting, smearing or other damage to the printed image as caused by water or moisture. The encapsulating and resulting protective layer can also provide additional benefit such as the reduction of static charge accumulation, UV blockage, glare reduction, and can act as a barrier to gaseous and liquid agents. The encapsulation of the printed image lengthens the lifetime of the image by protecting the image from color aging as caused by the bleaching of the colorant by chemical and radiation mechanisms.
It is another object of the present invention to provide methods for decreasing the printed resolution of images without loss of perceived image quality, thus reducing the amount of ink used, accelerating the speed of printing and reducing the time required for the print to dry.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.
The perceived quality of the pixilated printed output including a printed substrate and a spot of ink composed of colorants deposited and absorbed into the top surface of the substrate is enhanced by methods of the present invention. A porous or matrix layer is first applied as a coating on a top surface of the printed substrate. The matrix layer is partially absorbed into the substrate. A reflow agent is then applied to the coated substrate for solubilizing the colorants of the deposited ink so that the colorants diffuses into adjacent regions in the upper part of the printed surface of the substrate and/or into the porous coating. The matrix layer is semi-permeable to the variety of the reflow solvents, and is impermeable to the deposited ink.
The reflow agent can be applied on top of the matrix layer in a liquid form in an amount such that the colorants will not diffuse out of the matrix layer. Alternatively, the reflow agent can be applied in a vapor form, which diffuses and condenses into the matrix layer and the printed substrate.
The reflow agent also can be applied from a bottom surface of the substrate and in a liquid form. In this case, the matrix layer needs not be semi-permeable to the reflow agent.
Alternatively, the reflow agent can be applied into the matrix layer prior to applying the matrix layer on top surface of the substrate. The reflow agent can be mechanically mixed into the matrix layer as a sol/gel solution or as an adhesive coating of a tape. The reflow agent also can be microencapsulated in the matrix layer and is released from the matrix layer after the matrix layer is deposited on the top surface of the substrate, which leaves vacuoles in the matrix layer.
The reflow agent may be selected to non-black colorants only, thus it does not solubilize the black colorant.
The diffusion of colorants is terminated by either evaporating or solidifying the reflow agent, thus the matrix layer acts as a coating of the printed substrate for protection against damage and aging.